My cat pee's on my chair, and now my bed. Help?

My cat is a 2 1/2 year old neutered male. We live in an apartment, where I believe a stray cat visits to pee on our porch. My cat has begun peeing on the chair near the window where the stray cat visits. We have thrown the chair out, and now if given the opportunity he pees on my bed, he is not allowed in our room without supervision. I am afraid he will pee on our brand new sofa, and that will cause a serious issue. How can I stop this behavior, I believe he is upset that he now shares our apartment with my room mates two cats, however her cats are kept in her room, unless we are all at home, so his interaction with them is seldom. He does not have any medical issues, and I cannot get rid of him. We have more than enough litter boxes that are scooped daily. How can I make this stop?